This invention relates to thermal barrier coating systems such as used to protect some components of gas turbine engines and, more particularly, to the bond coat surface and the composition of the thermal barrier coating.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel- and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as a thermal barrier coating (TBC), or topcoat, of TBC systems used on gas turbine engine components. The TBC employed in the highest-temperature regions of gas turbine engines is typically deposited by electron beam physical vapor deposition (EBPVD) techniques that yield a columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic topcoat materials and the superalloy substrates they protect. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is usually employed. Bond coats are typically in the form of overlay coatings such as MCrAlX (where M is iron, cobalt, and/or nickel, and X is yttrium or another rare earth element), or diffusion aluminide coatings. A notable example of a diffusion aluminide bond coat contains platinum aluminide (NiPtAl) intermetallic. When a bond coat is applied, a zone of interdiffusion forms between the substrate and the bond coat. This zone is typically referred to as a diffusion zone. The diffusion zone beneath an overlay bond coat is typically much thinner than the diffusion zone beneath a diffusion bond coat.
During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine service, bond coats of the type described above oxidize to form a tightly adherent alumina (aluminum oxide or Al2O3) layer or scale that protects the underlying structure from catastrophic oxidation and also adheres the TBC to the bond coat. The service life of a TBC system is typically limited by spallation at or near the interfaces of the alumina scale with the bond coat or with the TBC. The spallation is induced by thermal fatigue as the article substrate and the thermal barrier coating system are repeatedly heated and cooled during engine service.
There is a need for an understanding of the specific mechanisms that lead to the thermal fatigue failure of the protective system, and for structures that extend the life of the coating before the incidence of such failure. The present invention fulfills this need, and further provides related advantages.
The present invention provides an approach for fabricating an article protected by a thermal barrier coating system, and articles protected by the thermal barrier coating system. The life of the thermal barrier coating system is extended under conditions of thermal fatigue by delaying the onset of the alumina scale interface failure mode and also reducing the delamination of the thermal barrier coating.
A method of fabricating an article protected by a thermal barrier coating system comprises the steps of providing an article substrate having a substrate surface, and thereafter producing a flattened bond coat on the substrate surface by depositing a bond coat on the substrate surface, the bond coat having a bond coat surface, and processing the bond coat to achieve a flattened bond coat surface. A thermal barrier coating is deposited overlying the bond coat surface. The thermal barrier coating comprises yttria-stabilized zirconia having a yttria content of from about 3 percent by weight to about less than 6 percent by weight of the yttria-stabilized zirconia, preferably from about 3.8 to about 4.2 percent by weight of the yttria-stabilized zirconia. The thermal barrier coating is preferably deposited by a physical vapor deposition technique such as electron beam physical vapor deposition, although other techniques may be used.
The article substrate preferably is a nickel-base superalloy, and most preferably is a component of a gas turbine engine. The bond coat may be a diffusion aluminide bond coat such as a platinum aluminide bond coat, or it may be an overlay bond coat.
The protective coating may be flattened without removing material from the protective-coating surface, as by peening the protective coating. Alternatively, the protective coating may be flattened by removing material from the protective-coating surface, as by polishing the protective coating. Desirably, the step of processing the protective coating produces a protective coating surface wherein an average grain boundary displacement height of the protective coating is less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably substantially zero, over at least about 40 percent of the surface area of the protective coating but more preferably over the entire surface area of the protective coating. Where the processing is accomplished by polishing, the average grain boundary displacement height may be substantially zero in the polished areas, where the polishing is to a mirror finish. In most cases, the step of processing the protective coating is performed after the step of depositing the protective coating is complete. In some cases, however, the steps of depositing the protective coating and processing the protective coating are performed concurrently. Additionally, it is preferred that at least about 40 percent, and more preferably all, of the surface of the protective coating is flattened to have a grain displacement height of less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably zero.
An article protected by a thermal barrier coating system comprises an article substrate having a substrate surface, a bond coat on the substrate surface, the bond coat having a bond coat surface with a grain boundary displacement height of less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably substantially zero, over at least about 40 percent, and preferably over 100 percent, of the grain boundaries. A thermal barrier coating overlies and contacts the bond coat surface. The thermal barrier coating comprises yttria-stabilized zirconia having a yttria content of from about 3 percent by weight to about less than 6 percent by weight of the yttria-stabilized zirconia. Features discussed above in relation to the fabrication method may be used in conjunction with the article as well.
The present approach addresses two major mechanisms of thermal fatigue failure in thermal barrier coating systems. The flattening of the bond coat surface reduces the tendency of the bond coat to form the convolutions that lead to spalling of the alumina scale that forms on the bond coat surface. The selection of the yttria-stabilized zirconia with low yttrium content reduces the tendency of the thermal barrier coating to fail and to debond from the alumina as a result of differential thermal strains and stresses during thermal fatigue cycling, and also reduces the differential thermal strains and stresses on the alumina/bond coat interface. As a result, failure of the thermal barrier coating system during thermal fatigue is delayed, improving its life.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.